22-05-2014, 03:19 PM
Linear Series Voltage Regulator NOTES
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Introduction
For most electronic equipment a DC power supply is generally preferred since, except for a start-up
transient, the supply ideally does not introduce any fiduciary timing dependence. However by and large
electrical power is generated and distributed with a sinusoidal waveform. Thus a power supply typically
begins with a rectifier to convert a sinusoidal input, e.g. 60 Hz for most U.S. consumer electronics, to a
rectified waveform. The supply is almost always a voltage supply as a practical matter; it is generally easier
and less lossy to maintain a voltage supply rather than a current supply in a standby condition, and to
operate it under varying load.
The unidirectional but varying rectified waveform is filtered in various ways to reduce the variation (the
'ripple' voltage) to an acceptable level. Nevertheless for many purposes even the filtered supply voltage
ripple variation often is unacceptably large, particularly within practical filtering limitations. Power line
variations, for example, are passed on to the rectified output. Moreover the Thevenin equivalent circuit for
the rectified and filtered power supply often involves a substantial 'internal' resistance, so that the terminal
voltage of the supply varies with the amount of current drawn because of the voltage drop across this
internal resistance. A 'voltage regulator' inserts additional electronics between the rectifier terminals and the
load primarily to reduce this terminal voltage variation, but also to provide other associated benefits.
Voltage rectification and filtering is discussed elsewhere. The objective of this note is to provide an
introduction to voltage regulator operation. The presentation will favor discrete element regulators for
illustration, although in fact it is only infrequently that a monolithic integrated circuit regulator would not be
preferable on both technical and economic grounds. It is simply pedagogical purpose that favors the
discrete illustration. A regulator involves several subcircuits performing separate functions, which are
coordinated to realize an overall purpose. Monolithic regulators are a sophisticated derivative of the
discrete circuit concepts, and do not differ markedly either in fundamental principle or basic circuit
concepts from their discrete counterparts. It is the system aspect that makes the discrete voltage regulator
of special instructional interest in the context of an associated introductory design project.
Zener Diode Shunt Regulator
Perhaps the simplest voltage regulator circuit is suggested by that
drawn to the right. VS and RS represent the Thevenin equivalent
of an unregulated power supply, feeding a load RL. To maintain
the load voltage constant a battery (idealized) is placed across the
load. The current supplied through VS is (VS-VB)/(RS+RB),
and is designed to be greater than the maximum value of the load
current VL/RL over the rated range of operation. The terminal
voltage of the extended supply is fixed by the properties of the (here idealized) battery. 'Load regulation',
i.e. changes in load current as RL varies, is provided by division of the supply current between the battery
and the load. 'Line regulation', i.e. changes in input voltage is accommodated by an increased voltage drop
across RB. This circuit is an example of a shunt regulator; the circuit element making the regulation
adjustment shunts the load
Overall 'Series-Pass' Voltage Regulator
The circuit diagram drawn above puts all the pieces together in a (simplified) representative discrete device
series regulator configuration. Note that the collector-base resistor of the control device also is the collector
resistor of the 'error' transistor in the comparitor. The error signal is applied to the base input of the series-
pass transistor. Adjusting the sampling potentiometer changes the quiescent setting of the pass transistor
base voltage; thereafter fluctuations in the output voltage produce amplified corrective changes in the base
voltage.